This invention relates to the manufacture of microeletronic circuits, and more particularly to a method of improving adhesion of elements such as lids and electrical connections therein.
Microelectronic units typically include a metallized ceramic package in which one or more electronic chips are mounted on die pads. In the mounting process, hereinafter sometimes designated "die-attach", each chip is normally fastened in place by soldering, whereby a gold-tin solder on the chip contacts an adherent layer of metallic gold on the die pad. The solder and metallic gold are melted by heating at temperatures above 400.degree. C., typically about 420.degree. C., for a suitable period of time, typically 1-8 minutes, after which the chip is mounted and the gold is allowed to solidify.
The gold is itself deposited via electrolytic or electroless deposition on a layer of another metal which facilitates electrically conductive connection with other circuit elements by soldering. Nickel and cobalt, and especially nickel, are frequently used for this purpose; frequent reference to nickel will be made hereinafter, but it should be understood that cobalt can be substituted for nickel when appropriate.
Following die-attach, connections are made via wires passing through vias to provide electrical connection with other circuits or other parts of the same circuit and The wires are bonded and the package is hermetically sealed by means of sealing elements, typically of screen-printed solder and especially lead-tin solder, contacting the metallic gold on the package. The resulting assembly is heated to just above the liquidus of the solder, ordinarily to about 370.degree. C., for a few minutes.
For reasons of economy and simplicity, the nickel and gold layers are ordinarily applied to all required areas of the metalized ceramic package in a single operation, which is followed by die-attach and subsequently by sealing. Thus, the entire package is exposed to the relatively high temperatures of die-attach. At these temperatures, there is frequently a substantial amount of diffusion of nickel through the gold layer to the surface thereof, where it is oxidized to nickel oxide. The latter severely impairs dissolution of the gold in the sealing elements into the solder of the lid, causing incomplete sealing.
Various strategies have been employed to minimize the effects of nickel diffusion through the gold layer. One is to increase the thickness of the gold layer to a point where nickel diffusion to the surface thereof will be insignficant during the die-attach process. This generally requires gold layers of extreme thickness, since it is frequently found that a layer of three times the normal thickness is not adequate to completely suppress nickel diffusion to the surface. This strategy, therefore, is very expensive.
A second approach is to interpose between the nickel and gold layers a layer of a third metal which inhibits nickel diffusion and which does not itself diffuse as rapidly as nickel. Cobalt (when the first metal layer is actually nickel rather than cobalt) and platinum are typical metals of this type. Such interposition, however, obviously involves still another plating step and the use of additional expensive and strategic materials. Moreover, such metals as cobalt are only marginally less capable of diffusion than nickel.